The present invention relates to a method for producing a toner for developing electrostatic images, which is used in electrophotography, electrostatic recording, etc.
A developer which is used in electronic duplicators, etc. is, for example, once attached to an image carrier of a photoreceptor having thereon an electrostatic image in a developing step, then transferred from the photoreceptor onto copying paper in a transferring step and thereafter fixed on the surface of the copying paper in a fixing step. As the developer to be used in this process for developing the electrostatic image formed on the surface of the image carrier, there are known a two-component developer comprising a carrier and a toner and a one-component developer (magnetic toner, non-magnetic toner) not containing a carrier.
The toner to be in the developer includes a positively-charged toner and a negatively-charged toner. As materials that make a toner positively charged, there have heretofore been known nigrosine dyes, quaternary ammonium salts, etc.; and as materials that make a toner negatively charged, there have heretofore been known charge-controlling agents such as metal-containing dyes, etc., coating agents that make a carrier predeterminately charged, etc.
One example of a conventional flowchart for producing a toner is shown in FIG. 1. According to this, predetermined amounts of raw materials such as resins and colorants are first weighed and mixed, then melted and kneaded in a kneader, and thereafter cooled, pulverized and classified. Next, the thus-classified toner is stirred and mixed with additives added thereto, coarse particles are removed by sieving, and the toner thus prepared is filled into a container.
The fine powder (fine toner powder) separated in the classification step has heretofore been re-used by recycling a predetermined amount of the powder to the step of mixing raw materials in consideration of environmental problems, the production costs, etc., as so described in Japanese Patent Laid-Open No. 5-34976. However, the conventional method of recycling such a fine toner powder had various problems in that the resin molecules existing in the fine toner powder are again cut while the fine toner powder is again melted and kneaded in a kneader with the result that the thus-cut resin molecules have a lowered molecular weight and worsen the fixing ability of the toner to cause toner hot offset, etc. and that the thus-cut resin molecules have lowered mechanical strength and worsen the durability of the toner. In particular, the lowering of the molecular weight of the resin molecules in the fine toner powder was often a serious problem especially when resins containing crosslinked components or non-crosslinked resins having two or more molecular weight peaks in its gel permeation chromatogram for low-molecular components and high-molecular components in the resins were used. Moreover, the addition of the fine toner powder to raw materials is unfavorable since the raw materials containing the powder are inferior to raw materials not containing it. The reasons are as follows: (a) It is difficult to uniformly mix the raw materials and the powder with the result that the composition of the toner to be finally obtained is not uniform. (b) Since the bulk density of the raw materials containing the powder is lowered, the introduction of the raw materials into a kneader is difficult when the raw materials are continuously fed thereinto with the result that the producibility of the toner is lowered. In addition, it is difficult to well shear the raw materials during kneading them with the result that the additives added thereto cannot be dispersed sufficiently. (c) When the raw materials mixed along with the fine toner powder are once stored in a container and then continuously fed from the container into a kneader, the fine toner powder is separated from the raw materials in the container due to the difference in the particle size and the specific gravity therebetween with the result that the composition of the toner being produced becomes uneven during the course of its production. As a result, the quality of the toner to be finally obtained is worsened.
Recently, in particular, the following problems with duplicators shall be taken into consideration. (a) High-speed duplicators are desired and the preparatory time for copying with duplicators is desired to be shortened. In view of these requirements, the binder resin to be in a toner for such duplicators is designed to have a lowered flow melting point in order to improve the fixability of the toner. Therefore, the mechanical strength of the binder resin is low and the toner materials containing the binder resin are easily over-pulverized to give much fine powder. (b) High-quality duplicates are desired. If toner particles having a small particle size (about 3 to 9 .mu.m) are desired to be obtained in order to satisfy the requirement of obtaining such high-quality duplicates, the efficiency for classifying such small toner particles is lowered and much fine toner powder is formed during the classification. As a result, the amount of the fine toner powder to be recycled during the production of the toner is increased. If so, it is difficult to attain the high producibility of the toner without lowering the quality of the toner.
Therefore, the first object of the present invention is to provide a method in which resin molecular weight variation is lowered and a toner having improved fixability and durability can be obtained. The second object is to provide a method for producing a toner having additives well dispersed therein and having a uniform composition. The third object is to provide a toner hardly depending on the environment while having excellent storage stability. The fourth object is to provide a toner having good charging and imaging characteristics and good durability, with which high-quality images can be obtained even by continuous copying operation. The fifth object is to provide a method for producing a toner with high producibility even though fine powder formed during the production is recycled.
We, the present inventors have assiduously studied and, as a result, have found that a toner with improved quality can be produced by recycling a fine toner powder formed during the production to the kneading step. On the basis of this finding, we have completed the present invention.